Image-reading devices cause an irradiation device to emit light to a document, receive reflected light of emitted light from the document, and generate image data containing information represented on the document. As light sources of the emitted light, xenon lamps and halogen lamps are widely used, but in recent years, in accordance with the tendency of energy saving, those lamps are being replaced with light emitting diodes (LED: Light Emitting Diode) (see, for example, Patent Literatures 1 and 2).
In general, a folding line and a wrinkle of a document, and a nearby portion to the opened portion between pages when the document is a book sometimes appear as shadows in an image represented by image data generated by image-reading devices. The image sensor disclosed in Patent Literature 1 and the bifurcation linear light source device disclosed in Patent Literature 2 emit illumination light to a document in various directions in order to suppress a generation of such shadow.
The image sensor disclosed in Patent Literature 1 includes a bar-shaped light guide that propagates light from LEDs disposed at an end. The light guide includes first and second light scattering layers that cause propagated light scattered and reflected, a first light outgoing area that emits scattered and reflected light in the first light scattering layer, and a second light outgoing area that emits scattered and reflected light in the second light scattering layer. Light (primary light) emitted from the first light outgoing area is directly emitted to an irradiation portion of a document in an oblique direction. Light (secondary light) emitted from the second light outgoing area is reflected by a reflector, and is emitted to the irradiation portion of the document in an oblique direction. The secondary light is emitted to the irradiation portion of the document from the opposite side to the primary light relative to a lens that converges light reflected from the irradiation portion of the document.
According to the image sensor disclosed in Patent Literature 1, in order to balance the luminous radiance between the primary light and the secondary light so as to suppress a generation of shadows due to a folding line and a wrinkle, the second light scattering layer is wider than the first light scattering layer.
According to the bifurcation linear light source disclosed in Patent Literature 2, a reflection mirror (reflector) has a cross-sectional area in an oval shape or in a radial shape as viewed from the lengthwise direction of the light guide. Accordingly, the widespread width of light (secondary light) reflected by the reflection mirror and emitted to a reading face of a document becomes substantially same as that of light (primary light) directly emitted to the reading face of the document.
In addition, Patent Literature 3 discloses a shading correction method of emitting direct light from a light source and reflected light through a reflector member in opposing oblique directions to a document, thereby adjusting the illuminance distribution in a sub scanning direction when the light source is mainly xenon lamps.